Four major classes of endogenous retroviruses have been identified in the genus Mus. These include two subclasses of endogenous type C virus, a novel class of endogenous retrovirus (designated M432) and type B endogenous virus. Using viral reverse transcripts we have estimated the number of endogenous retroviral related sequences in various members of this genus. Taken together, the total complement of retroviral sequences represents a significant portion (approximately 0.04%) of the mouse cellular genome. Each of the classes of viral sequences we have studied (C-I, C-II, and M432) appears to be conserved to different extents in different rodent species, suggesting that different evolutionary pressures affect the rate of divergence of each of these sets of viral genes.